Many clinical chemistry assays are performed using reagents that form colored indicators. In such assays, the intensity of the color of the indicator is correlated to the concentration of analyte in the fluid sample being measured.
Exemplary of such assays is the analysis of glucose from a blood sample by utilizing a reagent that forms Prussian Blue (or Turnbull's Blue) as a colored indicator. The reagent may include the enzyme glucose oxidase (GOD), and potassium ferricyanide and ferric sulfate for the formation of Prussian Blue. The reaction that generates Prussian Blue may be depicted as follows (Scheme I): ##STR1##
Accordingly, the more Prussian Blue that is formed by this reaction, then the more glucose is present in the blood sample being analyzed.
However, a particular problem with reagents that are used to form the Prussian Blue indicator, and other reagents that are used to form indicators that involve ferric ion complexes, is stability. Such reagents, when in liquid form, are unstable to light and heat, and, when in lyophilized form or included in a film, are additionally unstable to humidity. In the case of a reagent useful for forming the Prussian Blue indicator in a glucose assay, the instability is manifested by premature formation of Prussian Blue ("the blank reaction") in the reagent prior to addition of a blood sample containing glucose.